A Local Area Network such as an Ethernet typically comprises a plurality of stations (also known as Data Terminal Equipments or DTE's) connected to a hub. In one conventional Ethernet arrangement known as 10 base T, a station and the hub are connected by two unshielded twisted pairs (UTP's). Each of the two pairs transmits unidirectionally. One of the two pairs is used by the station only to transmit data to the hub and is used by the hub only to receive data from the station. The other of the two pairs is used by the station only to receive data from the hub and is used by the hub only to transmit data to the station.
The 10 base T system uses a collision detection algorithm which operates as follows: A station detects the start of a collision by detecting energy (i.e. a carrier) on its receive only pair while the station is transmitting on its transmit only pair. The hub detects the start of a collision with a particular station by detecting the presence of energy on the receive only pair connected to the particular station while the hub is transmitting on the transmit only pair connected to the particular station.
After detecting the start of a collision, the station sends a jam signal for a specified time period on its transmit only pair and then goes quiet. After detecting the start of the collision, the hub sends a jam signal on the transmit only pair leading to each station connected to the hub. The hub keeps sending the jam signal on all its transmit only pairs until the hub detects no energy on any of its receive only pairs, then the hub goes quiet. Transmissions between the stations and the hub then resume in accordance with the conventional exponential backoff algorithm specified by the IEEE 802.3 Standard.
This collision detection algorithm is advantageous in many low bitrate situations because collision detection is simple and fast. All that is required by the station or hub to detect a collision is to detect energy on a receive only pair while a transmission is taking place.
The 10 base T Ethernet generally operates up to 10 Mbps. Recently efforts have been directed to increasing the speed of Ethernets to up to 100 mbps.
In one high speed Ethernet four unshielded twisted pairs are used to connect the hub to each station (see e.g. J. R. Rivers "PMD Proposal Utilizing Four Category 3 UTP Wirepairs, 5B6B Encoding and Low Frequency Collision Detection" IEEE 802.3 Higher Speed Study Group, Wakefield, Mass., May, 1993, and Howie Johnson "CSMA/CD Using 2 or More Pairs" 802.3 Higher Speed Study Group, Wakefield, Mass., May 1993). All four of the pairs are used bidirectionally to transmit data from the station to the hub and also to transmit data from the hub to the station. A collision is detected as follows: Consider the case where the station is transmitting data at a high frequency on all four pairs to the hub and then the hub starts to transmit on all four pairs. On one of the pairs the hub starts its transmission with a low frequency Start Of Carrier (SOC) signal. The station continuously monitors this pair. The difference between any signal received on this pair from the hub and the signal transmitted on this pair by the station is continuously obtained. The difference signal is then filtered by a low pass filter whose pass band corresponds to the frequency band of the SOC signal. Any energy in the SOC band will be detected by the station as a collision. Collision detection at the hub takes place in a similar manner. Consider the case where the hub is transmitting high frequency data on all four pairs: When the station starts transmitting, it sends a low frequency SOC signal on one particular pair, which particular pair is continuously monitored by the hub. The hub continuously obtains the difference between any signal received from the station on this particular pair and the data signal it is transmitting on this particular pair. The difference signal is filtered by a low pass filter whose pass band corresponds to the low frequency SOC signal. Any energy in the SOC band will be detected by the hub as a collision.
This technique for using multiple pairs between each station and the hub to achieve a fast Ethernet has significant shortcomings. Specifically, the collision detection technique is unsatisfactory because it is too slow and too complex. The need to obtain a difference signal and then filter the difference signal to detect a collision causes the time required to detect a collision to be too long, thereby degrading the performance of the entire network. The time required to detect a collision in the above described fast Ethernet technique is much longer than the time required to detect a collision in the 10 base T technique. Furthermore, in comparison to the 10 base T technique, the above described Fast Ethernet technique requires additional circuitry in the form of a hybrid to obtain the difference signal and a low pass filter to filter the difference signal.
In view of the foregoing, it is an object of the present invention to provide a fast Ethernet in which multiple pairs connect each station to the hub, but in which a simpler collision detection technique is utilized.